Chuck and Ryan's Build Tips

[ceslaw]

Covering the wings

I now know several thing: that the Carbon Cub wing is also called a “bad boy”; when Steve and Armando take their break; that it is way bad to get a wrinkle when gluing down the fabric on the wing tip; and that Mitch does not joke about Armondo’s height. I can’t count the number of times I have watched the wing covering video, but it has been really helpful and each viewing typically answered another, perhaps subtle, question.

1. The wing in the video does not have a landing light or extended fuel so a few tips follow. Before covering the top of the wing, Poly Tak is placed on the inner rib, and forward and aft locations where the fabric will eventually be cut out to expose the fuel tanks. A liberal ( I hate that word ) coat is applied and left to dry. I taped off the areas to assure a perfectly straight glue line, removing the tape before the glue was completely dry.

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Glue is NOT pre-applied in the notch with the nut plates on the forward and aft edges in which the cover is placed.

After the first 250 degree shrink, the glue that had been pre applied is activated with MEK. After it dries the fabric over the tank is cut open, leaving about a 3/8 inch piece of fabric around all four edges. We cut notches in the fabric to avoid interference with the nut plates on rib number three.

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Gluing fabric on the underside of the third rib is a challenge made easier by using a flat piece of metal. On the first rib we simply trimmed it flush using a razor blade since the fabric is under no tension.

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The fabric needs to be tightly secured to the notch where the cover sets. A tongue depressor was used to make sure the fabric was tightly secured to the notch as the Poly Tak dried.

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2. The same technique was used on the landing light.

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After the 250 degree shrink, the fabric over the landing light opening was marked with a pencil line about ¼” from the edge all the way around. The line was then dabbed with Poly Tak to prevent it from fraying. An Xacto or razor blade was used to cut along the line, keeping it as straight as possible. This was then glued in place to create a neat, straight edge.

3. After repeated openings of the Poly Tak can it did not want to seal well due the accumulation of glue. When we started the second can a small section of Saran Wrap placed over the opening before the lid was placed to seal without the mess. After a few openings a new piece is used.

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4. After the final shrink we noticed the second, bottom false rib had bowed slightly. If left, the row of rivets along it length would not have been in a straight line when viewed from front or rear. Accessing the rib in the space between the fuel tanks. we used tongue depressors to gently push it back in place, holding them with a folded towel, and making sure the tongue was not in direct contact with the fabric. We will leave this ‘crutch’ in place until it is Poly Brushed and riveted.

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[ceslaw]

Screw up

After gluing the grommet and applying the doily for the aileron cable cover on the top of the wing, I discovered it was about an inch off. So the issue was how to remove and replace them.

I brushed a liberal (I hate that word) coat of Poly Brush reducer over the doily to soften the Poly Brush and it quickly came loose making it easy to peel away. MEK was not used because it is a bit too aggressive. The plastic grommet was also loosened with the reducer brushed along the edge and popped off easily.

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A bit of clean up with the reducer and only a faint shadow of the misplaced items remained.

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A new doily was made from a scrap of light weight fabric. It is nice to know that screw ups can be fixed.

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[Cubrath]

Chuck,

You caught it at the perfect time. On my first cub I made that same mistake and I didn't catch it until after the wing was finished. The repair was much more involved at that point!

MR

[ceslaw]

Taping wings

Having never covered a plane before, it has quickly become apparent that once the basics are mastered the technique attendant to the details becomes critical. I can’t say if the following is right or wrong, but it worked for me.

1. Covering the nav lights. There are two small fabric pieces to cover the top and bottom of the navigation lights. The bottom went on first. A bit of Poly Brush was used to hold down just the inner edge. Once dried the iron was used to shape the remaining section around the nav light, then Poly Brushed.

The same technique was used to apply the top piece, but after ironing it to shape it was trimmed so the end seam was along the center line.

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2. Aft rib tip. A three inch tape was folded down the center to establish a center line. The aft end was held in place with a clamp. The tape was then pulled tight to cause the tape to tend to fold flat and additional clamps applied to about six inches of the nav light. Next the tape was ironed along the outer edges at about 250^ until it laid flat. Finally the Poly Brush was applied, removing clamps as it was glued. Once it dried sufficiently the clamps were removed and the end by the nav light was trimmed with a rounded end using a pattern and Poly Brushed in place.

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3. Forward tip. The forward tip can’t be clamped because it is too darn thick. So the tape was held at the forward leading end with a dime size dab of Poly Tack. Then after it set up another dime size dab was applied about three inches away, pulling it snug. After several dabs of Poly Brush were applied and dry, an iron was used to shape the tape and Poly Brush applied.

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[ceslaw]

Taping.

Using fabric rivets rather than stitching requires that as little tape as possible be used so that the Poly Brush can adhere directly to the ribs. We used a combination of green masking tape and fabric tape.

a. Several lengths of ¾” green masking tape were placed on a glass cutting board and then using an X-Acto knife perpendicular cuts were made ¾” apart. With these precut sections of tape it was much quicker putting tape over the rivets on the leading edge and other locations and the sections of tape were all exactly the same.

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b. No tape was applied on the aileron valley.

c. Tapes were applied sparingly:

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[ceslaw]

Covering the Wings Revisited

1. Filler piece. In the award winning Mitch Travis video, Armando described a technique for gluing the filler piece of fabric near the wing tip. He folded the forward tab outward toward the wing tip, using a plastic wrapped piece of metal as a clamp to hold it in place, then removed the clamp after it was dried. I discovered that when heat shrinking the fabric this edge has a tendency to pull away, requiring that a small patch be added to smooth the surface.

On the second wing, the tab was folded inward toward the wing root. The tab was covered with the top and bottom layers of wing fabric. The edge of the tab disappeared beneath those layers and did not pull away when shrinking the fabric.

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2. Fabric overlap. The video specifies a 1 ½ inch overlap of the fabric on the leading edge. This is not sufficient if the fabric will be cut away over the fuel tank for an aluminum cover. Two inches is needed to assure enough glue area along the leading edge of the fuel tank opening. Note that only about a half inch along the leading flat surface is glued initially; the recessed area is glued after the fabric is shrunk to 250^ and cut away.

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[ceslaw]

Aileron valley.

The concave area behind the aileron can be problematic. It needs to be solidly glued with no bubbles. To assure a solid seal we deviated from the video in the following respects:

a. No seam tape or masking tape was applied on the seam or over the rivets. Since the fabric is not shrunk in this area and it will end up with a total of three layers of fabric, I did not consider this to be an issue. This will assure maximum area of adhesion

b. A coat of Poly Tak was brushed over the aluminum and permitted to dry thoroughly before any fabric was applied. Sort of a ‘primer’ coat.

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c. After the top fabric was secured along the top edge per the video, slits were cut at approximately 17” intervals with a straight scissors (not a pinking shear) so that smaller sections could be glued carefully one at a time both in the aileron valley and the flap area.

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d. The fabric was glued in place by brushing a heavy coat on the ‘primed’ metal, and then pressing the fabric in place. We thinned the Poly Tak slightly with about one part MEK to five parts glue. Thinning it really made it go on smoother and it sealed it tightly to the ‘primer’ coat of Poly Tak. Poly Tak was not brushed on top of the fabric at this time. Extra effort was made to assure the fabric was smoothed in place without any bubbles, using a plastic hotel door key as a squeegee. (I always keep those hotel keys. They come in handy for projects like this.)

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e. After the top fabric dried, a coat of thinned Poly Tak was brushed over the fabric from the top of the wing to serve as a ‘primer’ coat for the fabric overlap from the wing bottom. After it was dry, the same technique was followed to glue the fabric from the wing bottom to the valley

f. The area was carefully inspected after it was thoroughly dry. Shining a light at an angle may reveal bubbles not otherwise visible. MEK was rubbed in those areas where we had the slightest concerns.

g. An iron was NOT used anywhere in the aileron valley. Any shrinking of the fabric could create a bubble that will be hard to repair.

It really came out well: smooth with no bubbles or bulges.

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[ceslaw]

False Rib

I thought I would post a couple of items regarding construction of the wings, even though it has been a year since this part of the build was done. Perhaps it will be of some benefit to current builders.

The false rib in the number 2 position (used with the extended fuel option only) required a little extra attention if one intends to use fabric rivets rather than stitching.

The false rib, part number SC31048, has a front and rear based on the hole spacing. Compare the spacing of the fabric rivet holes with another rib already in place and it will be obvious. The first and second holes are spaced closer on the forward side.

The manual directs that the cutouts on the top side be centered fore and aft to provide clearance for the short hoses connecting the two fuel tanks. Set the false rib in place and mark a line adjacent to the spar on the rib for future reference. Then place the false rib next to another rib properly centered fore and aft and compare the location of the fabric rivet holes. All the fabric rivet holes were 7/8” too far aft on our false rib. (The use of fabric rivets by CC is a recent change. I suspect the issue regarding hole location in the false spar will be quickly addressed by CC).

The correct location of the holes was marked and drilled with a #30 drill bit. The original holes were marked with an “X.” Note this is only an issue if fabric rivets will be used. For conventional wing stitching this would not be necessary. Indeed this is purely a cosmetic issue since I doubt many would even notice the fabric rivets being off 7/8 inch on this one rib.

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[ceslaw]

CherryMAX Rivets

Stainless steel pull rivets, squeeze rivets and CherryMAX rivets are used on the wings. The first two did not create any issues but the third did.

The CherryMAX rivets have to be pulled perpendicular to the surface or they will not seat properly. What makes this problematic is one can pull a CherryMAX, the stem pops off in the usual manner, and looks like all is well. But it may not be seated properly. The first clue there is a problem is to check carefully to see if the stem extends to the surface of the hole. The better clue is to look at the back side. If there is a stem sticking out you have a problem and it did not seat properly.So how to you see the back side of a rivet in a tight place such as the web of the spar? Those little round mirrors on a rod work well. Like a dentist would use.

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The CherryMAX rivets placed on the upper holes on the inboard and outboard aileron supports are a challenge because the angle of the supports makes it nearly impossible to get a squeeze riveter perpendicular to the surface. Here are a couple of tricks.

First, start grinding. A ten dollar riveter from Rural King was the starting point. A power grinder was used to cut away material until the riveter would fit in close to perpendicular. So much material was removed that the fact it could still pull a rivet was a bit surprising.

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Second adding a spacer between the rivet head and the riveter may help. An aluminum wire stop works perfectly. It may need to be filed down a bit so that there is enough of the rivet stem to catch the teeth in the riveter.

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If the rivet does not seat properly there likely won’t be a stem visible in the center. The good news is it is easy to drill out without that pesky stem at the surface of the rivet head. Drill just deep enough to pop off the head of the rivet and then use a punch the same size as the hole to push the remaining rivet out. Done right the next rivet should fit tight.

[ceslaw]

Rear Spar Reinforcement

This is about a really small detail that in the final analysis makes no difference. But if one is fanatical about holes lining up, or shall I say one hole lining up, I will pass the information along.

A long flat reinforcing strip is riveted to the aft spar to reinforce it, hence the name “rear spar reinforcement.” It is a husky strip with lots of holes. Near the center of this strip a pulley bracket is mounted, after the reinforcing strip is securely riveted in place. The manual gives very precise dimensions on locating the reinforcing strip, but then adds to the dimension “+ 1/16, -3/16”. Now I am not a math major, but that looks like a quarter inch of variation, so how do you decide where to place it?

I will give you the answer.

First, put the pulley together (temporarily) and set it in its appropriate location on the aft spar, with a guide string that will run perfectly parallel to the aileron center support. (You are essentially skipping ahead a few steps in the manual, but don’t worry, we will get back to the reinforcement strip).

Second, note the directions clearly state that one of the holes should NOT be drilled through the reinforcement strip and riveted to the spar. Put a nice black (or the color of your choice) “X” on that hole.

Third, set the reinforcement strip in place under the pulley bracket, lining up the hole in the pulley bracket that is “match drilled” with the hole in the reinforcement strip that you just marked with the “X”. Make sure you keep the guide string exactly parallel to the aileron support.

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You now have the correct location for the reinforcement strip. Clamp it in place before it can get away. If you now measure per the directions I suspect you will find the spar reinforcement is at the minus 3/16 range of the specified dimension.

Now you can go back to all the drilling and riveting per the instructions. If, like me, you just used the reference dimensions and did not realize the significance of what was to come several steps later with aligning the pulley bracket, don’t despair. It just means you won’t be match drilling that hole but will have to drill a new hole. There is enough space to do it and no one will ever know. At least no one would have known if I had not done this post.